How to disable or reset the anti theft system??

I have a 2004 Chevy Classic. I just bought it last year, 174,000 miles. The first time the anti theft system malfunctioned I thought it was my fuel pump, starter, or out of gas. I know nothing about cars! After noticing my blinking theft system light, a quick googled search showed me how to reset the system. (I used the-- turn key to "on" leave for 10 minutes until light stops blinking, turn back to "off" position, then start. worked like a charm)I've gotten used to running out to start my car 10 minutes before actually having to leave. (this happens a lot!) Last weekend my theft system light came on, but was not flashing and would just stay on. I tried the 10 minute reset, the lock/ unlock, something about messing with the radio power button. Nothing worked! I decided to let it be and took my husbands car. After a few hours of not trying, my husband went out and started messing with the power locks, and radio, put the key in the ignition and the damn thing started right up!! Again today this happened. After spending 2 1/2 hours in the parking lot of work (tried disconnecting the battery, leaving it for 15 minutes then hooking it back up) I had my husband make the 30 minute drive to pick me up.

Okay! after that long winded explanation I need to know how to get my car started!! I'm fed up and just want to sell it! Any suggestions on anything I haven't tried yet. I'm very confused as to why my light went from blinking when this happened, to a solid light??